Before the mountains were brought forth, or ever you had formed the earth and the world, from everlasting to everlasting you are God (Psalm 90:2)
Children sometimes ask, “Who made God?” The clearest answer is that God never needed to be made, because he was always there. He exists in a different way from us: we, his creatures, exist in a dependent, derived, finite, fragile way, but our Maker exists in an eternal, self-sustaining, necessary way—necessary, that is, in the sense that God does not have it in him to go out of existence, just as we do not have it in us to live forever.
We necessarily age and die, because it is our present nature to do that; God necessarily continues forever unchanged, because it is his eternal nature to do that. This is one of many contrasts between creature and Creator. God’s self-existence is a basic truth. At the outset of his presentation of the unknown God to the Athenian idolaters, Paul explained that this God, the world’s Creator, “is not served by human hands, as if he needed anything, because he himself gives all men life and breath and everything else” (Acts 17:23-25). Sacrifices offered to idols, in today’s tribal religions as in ancient Athens, are thought of as somehow keeping the god going, but the Creator needs no such support system. The word aseity, meaning that he has life in himself and draws his unending energy from himself (a se in Latin means “from himself”), was coined by theologians to express this truth, which the Bible makes clear (Ps. 90:1-4; 102:25-27; Isa. 40:28-31; John 5:26; Rev. 4:10). In theology, endless mistakes result from supposing that the conditions, bounds, and limits of our own finite existence apply to God.
The doctrine of his aseity stands as a bulwark against such mistakes. In our life of faith, we easily impoverish ourselves by embracing an idea of God that is too limited and small, and again the doctrine of God’s aseity stands as a bulwark to stop this happening. It is vital for spiritual health to believe that God is great (cf. Ps. 95:1-7), and grasping the truth of his aseity is the first step on the road to doing this.
Adapted from Concise Theology: A Guide to Historic Christian Beliefs by J.I. Packer